I have done a small board based on a PIC24FJ64GA004, a MRF24J40 2.4GHz radio and a one-wire interface. The PIC wakes up about once every 5th minutes, takes a temperature reading from a string of temperature sensors on the one-wire interface and sends the measurement results to a coordinator over the radio..

The unit is powered with 2 AA batteries and I expect these to last about one year.

The other day when I was at the store I saw a set of solar powered garden lights at a very reasonable price. They are supposed to be charged by the sun during the day and light a LED during the night. So I picked it up just to see how it was working, hoping to be able to use the solar panels as power supply for my radio unit. It turns out that the solar panel is directly connected to a small 1.2V, 80mAh NiMh battery. The battery is then connected to a a small circuit board via an on/off switch. The board has a LED and some circuitry under a blob, probably sensing the voltage from the solar panel to determine if it is day or night.

My powersupply is made to work with 2 cells so I took the NiMh batteries and the solar panels from 2 units and connected these in series. Still with each solar panel directly connected across each battery. I then connected the 2 NiMh batteries to my circuit instead of the 2 AA alkaline batteries.

I have placed the unit indoors behind a window where the solar panels are exposed to sunlight (through the window) a couple of hours every morning (if the sun is shining). When I measure the battery voltage in the morning before the sunlight hits the panels, the voltage is about 2.4-2.6V over the two batteries and after a couple of hours charging by the sunlight, the voltage is as high as 2.75V. The circuit is working very well and has been for about a week now. The 80mA batteries should be able to keep the unit working for a couple of days even without any charging.

Now, to my question: Is it safe to just connect the small solar panels directly across the NiMh batteries just like this (as it also was in the original circuit)?

The fully charged voltage of 2.75V seems a little high for the batteries. Could this reduce the lifetime for the batteries?

> Hi,
>
> I have done a small board based on a PIC24FJ64GA004, a MRF24J40 2.4GHz radio
> and a one-wire interface. The PIC wakes up about once every 5th minutes, takes
> a temperature reading from a string of temperature sensors on the one-wire
> interface and sends the measurement results to a coordinator over the radio.
>
> The unit is powered with 2 AA batteries and I expect these to last about one
> year.
>
> The other day when I was at the store I saw a set of solar powered garden
> lights at a very reasonable price. They are supposed to be charged by the sun
> during the day and light a LED during the night. So I picked it up just to see
> how it was working, hoping to be able to use the solar panels as power supply
> for my radio unit. It turns out that the solar panel is directly connected to a
> small 1.2V, 80mAh NiMh battery. The battery is then connected to a a small
> circuit board via an on/off switch. The board has a LED and some circuitry
> under a blob, probably sensing the voltage from the solar panel to determine if
> it is day or night.
>
> My powersupply is made to work with 2 cells so I took the NiMh batteries and
> the solar panels from 2 units and connected these in series. Still with each
> solar panel directly connected across each battery. I then connected the 2 NiMh
> batteries to my circuit instead of the 2 AA alkaline batteries.
>
> I have placed the unit indoors behind a window where the solar panels are
> exposed to sunlight (through the window) a couple of hours every morning (if
> the sun is shining). When I measure the battery voltage in the morning before
> the sunlight hits the panels, the voltage is about 2.4-2.6V over the two
> batteries and after a couple of hours charging by the sunlight, the voltage is
> as high as 2.75V. The circuit is working very well and has been for about a
> week now. The 80mA batteries should be able to keep the unit working for a
> couple of days even without any charging.
>
> Now, to my question: Is it safe to just connect the small solar panels directly
> across the NiMh batteries just like this (as it also was in the original
> circuit)?
>
> The fully charged voltage of 2.75V seems a little high for the batteries. Could
> this reduce the lifetime for the batteries?
>
> Regards /Ruben
>
>
> ===========================================
> Ruben Jönsson
> AB Liros Electronic
> Box 9124
> 200 39 Malmö Sweden
> http://www.liros.se
> Tel +46 40142078
> ============================================
>

Connecting the panels across the batteries via a Schottky diode is
safe enough in most cases. Your cells are low capacity, which
typically tolerate a large % of trickle charge compared to large
capacity cells. PV panel charge ability will be
low.````````````````````````````````````````````````````````````````````````````

The charging voltage is OK.
About 1.45V/cell is fully charged when measured while charging.

Largely indirect sunlight through glass will provide only a small % of
max charge rate. Direct sun is needed for max charge. Clean window
glass will reduce charge rate by 10%-20%.

On 5/7/2012 3:19 PM, Ruben Jönsson wrote:
>> I built a similar circuit that had a diode between the solar panel and
>> the batteries so they wouldn't try to feed power back to the panel and
>> it worked great.
>>
> Would that be a problem? Is this bad for the panels?
>
> The original unit did not have any diodes in series with the battery

I have some of those solar powered lights as well except mine connect the solar panel to a NiCd battery, rated for 600mah. The panels themselves output 3.45V with a 680 ohm load resistor. The board itself is kind of interesting, they have a 4 leaded to-92 style part that converts the battery voltage into enough voltage to run the LED lighting using a switching design, the only other part on the board is the inductor, looks like a small resistor. The 4 leaded switcher contains the diode and necessary hardware to control the charging and turn on/off the light.

Overall the panels used in the lights I bought were extremely poor quality . A better panel is the ones that Bgmicro has , they are encased in epoxy like material and produce 4.5V @ 200ma and only cost $3 each, seem to be much better quality.http://www.bgmicro.com/PWR1241.aspx

> Overall the panels used in the lights I bought were extremely poor
> quality . A better panel is the ones that Bgmicro has , they are encased
> in epoxy like material and produce 4.5V @ 200ma and only cost $3 each,
> seem to be much better quality.
> http://www.bgmicro.com/PWR1241.aspx

Pathlights have almost all been epoxy encapsulated until recently.
Quality varies widely.
Epoxy will not last more than a few years max if always outdoors and
can be far less.
Latest trend in low cost lights is PET plastic with heat laminated EVA
adhesive for bonding.
Best are flurocarbon plastics or purpose built silicone rubbers. (eg
Dow Corning PV6100).

Based on the google research I did before I built the circuit it was
consensus that it's probably not good for the panels to have power
pushed back into them at night so I went ahead and added the schottky
diode.

I don't think, within reason, that the voltage matters much, it's just
a matter of pushing electrons back into the battery. I'm sure the
batteries never got fully charged, but that didn't matter too much (in
fact was probably safer) for my application.

My panels were 3" * 4" and the design called for them to drive 5 LEDs
from dusk to midnight, and they did that just fine. I used a boost
converter LED driver to drive the LEDs and had a simple little PNP
transistor circuit to determine whether it was dark or not. Used 2@AA
NiMH.

>
>> I built a similar circuit that had a diode between the solar panel and
>> the batteries so they wouldn't try to feed power back to the panel and
>> it worked great.
>>
>
> Would that be a problem? Is this bad for the panels?
>
> The original unit did not have any diodes in series with the battery.
>
>> I don't know if that's strictly necessary, though, but it makes sense
>> intuitively.
>
> Yes but a diode voltage drop is pretty much in this circuit. Even for a
> schottky diode.
>
>>
>> DougM
>>
>
> /Ruben
>
> ==============================
> Ruben Jönsson
> AB Liros Electronic
> Box 9124, 200 39 Malmö, Sweden
> TEL INT +46 40142078
> FAX INT +46 40947388
> .....rubenKILLspam.....pp.sbbs.se
> ==============================
>

> Based on the google research I did before I built the circuit it was
> consensus that it's probably not good for the panels to have power
> pushed back into them at night so I went ahead and added the schottky
> diode.
>
> I don't think, within reason, that the voltage matters much, it's just
> a matter of pushing electrons back into the battery. I'm sure the
> batteries never got fully charged, but that didn't matter too much (in
> fact was probably safer) for my application.
>
> My panels were 3" * 4" and the design called for them to drive 5 LEDs
> from dusk to midnight, and they did that just fine. I used a boost
> converter LED driver to drive the LEDs and had a simple little PNP
> transistor circuit to determine whether it was dark or not. Used 2@AA
> NiMH.
>
> Caveat I live in Seattle, not Arizona :-)
>
> I can send you the schematic if you want to take a look at it.
>
> Thanks,
>
> DougM
>
> On Mon, May 7, 2012 at 12:19 PM, Ruben Jönsson <rubenspam_OUTrjjournal.net>
> wrote:
> >
> >> I built a similar circuit that had a diode between the solar panel and
> >> the batteries so they wouldn't try to feed power back to the panel and
> >> it worked great.
> >>
> >
> > Would that be a problem? Is this bad for the panels?
> >
> > The original unit did not have any diodes in series with the battery.
> >
> >> I don't know if that's strictly necessary, though, but it makes sense
> >> intuitively.
> >
> > Yes but a diode voltage drop is pretty much in this circuit. Even for a
> > schottky diode.
> >
> >>
> >> DougM
> >>
> >
> > /Ruben
> >
> > ==============================
> > Ruben Jönsson
> > AB Liros Electronic
> > Box 9124, 200 39 Malmö, Sweden
> > TEL INT +46 40142078
> > FAX INT +46 40947388
> > @spam@rubenKILLspampp.sbbs.se
> > ==============================
> >

> PV cells are DIODES. There is no need for an additional one to 'isolate' it
> from the battery.
>
> On Mon, May 7, 2012 at 5:23 PM, doug metzler <RemoveMEdoug.metzlerTakeThisOuTgmail.com> wrote:
>
>> Based on the google research I did before I built the circuit it was
>> consensus that it's probably not good for the panels to have power
>> pushed back into them at night so I went ahead and added the schottky
>> diode.
>>
>> I don't think, within reason, that the voltage matters much, it's just
>> a matter of pushing electrons back into the battery. I'm sure the
>> batteries never got fully charged, but that didn't matter too much (in
>> fact was probably safer) for my application.
>>
>> My panels were 3" * 4" and the design called for them to drive 5 LEDs
>> from dusk to midnight, and they did that just fine. I used a boost
>> converter LED driver to drive the LEDs and had a simple little PNP
>> transistor circuit to determine whether it was dark or not. Used 2@AA
>> NiMH.
>>
>> Caveat I live in Seattle, not Arizona :-)
>>
>> I can send you the schematic if you want to take a look at it.
>>
>> Thanks,
>>
>> DougM
>>
>> On Mon, May 7, 2012 at 12:19 PM, Ruben Jönsson <spamBeGonerubenspamBeGonerjjournal.net>
>> wrote:
>> >
>> >> I built a similar circuit that had a diode between the solar panel and
>> >> the batteries so they wouldn't try to feed power back to the panel and
>> >> it worked great.
>> >>
>> >
>> > Would that be a problem? Is this bad for the panels?
>> >
>> > The original unit did not have any diodes in series with the battery.
>> >
>> >> I don't know if that's strictly necessary, though, but it makes sense
>> >> intuitively.
>> >
>> > Yes but a diode voltage drop is pretty much in this circuit. Even for a
>> > schottky diode.
>> >
>> >>
>> >> DougM
>> >>
>> >
>> > /Ruben
>> >
>> > ==============================
>> > Ruben Jönsson
>> > AB Liros Electronic
>> > Box 9124, 200 39 Malmö, Sweden
>> > TEL INT +46 40142078
>> > FAX INT +46 40947388
>> > TakeThisOuTrubenEraseMEspam_OUTpp.sbbs.se
>> > ==============================
>> >
>> > --

> Connecting the panels across the batteries via a Schottky diode is
> safe enough in most cases. Your cells are low capacity, which
> typically tolerate a large % of trickle charge compared to large
> capacity cells. PV panel charge ability will be
> low.``````````````````````````````````````````````````````````````````````````
> ``
>
> The charging voltage is OK.
> About 1.45V/cell is fully charged when measured while charging.
>
> Largely indirect sunlight through glass will provide only a small % of
> max charge rate. Direct sun is needed for max charge. Clean window
> glass will reduce charge rate by 10%-20%.
>
>
>
> Russell

Thanks. Placing it behind the window will let me see how it performs in a non optimal placement. There is also a permanent shade roof above the window so it is only when the sun is low enough in the morning that the PV cells will see direct sunlight (through the window).

So far it gives me enough power for my circuit. We have a couple of days with cloudy weather ahead of us. Let's se how it copes with that.

Overall I am quite impressive with the power you can get from these cheap PV cells.

Now, I would like to only have one PV cell with a little bit higher voltage.. Would it be a good idea to charge the two NiMh cells from one PV cell (with the batteries connected in series)?

Or would it be better to redesign my circuit to use just one battery instead of two?

>> Based on the google research I did before I built the circuit it was
>> consensus that it's probably not good for the panels to have power
>> pushed back into them at night so I went ahead and added the schottky
>> diode.

A series reverse diode or equivalent is, alas, essential.
It's easy to not realise that they work "somewhat strangely" compared
to what may be logically expected.

Photovoltaic cells are diodes, but in photo voltaic generation mode
the Anode is positive relative to the Cathode so current flow when
delivering energy is against the flow that the diode will usually
permit. It makes sense when you look at the underlying theory but
Murphy had fun here.

The effect on the panels may be minimal depending on dissipation. They
may well survive OK in low energy systems.
BUT this will be fatal for battery stored energy.
When PV panel voltage falls below battery voltage the battery *will*
discharge into the panel.

When using low voltage low current panels such as in path lights, use
of a 1A Schottky at as low a reverse voltage as is sensibly available
is liable to pay (very) slight dividends.
A 1N5817 / SS12 diode is 20V rated (afair) and the 1N5818 / SS14 and
1N5819 are higher (30V, 40V, ...). They usually cost the same.
Often but not always the lower Vr diode will have usefully lower
Vforward - maybe more than 0.1V lower.
When you have lots of light and Vpv >> Vbattery it matters little but
as Vpv approaches Vbattery an extra 0.1V can help. Albeit, only
sometimes.

In the unlikely event that MPPT is being used, in a 3V system a 0.1V
drop may give an extra 3% energy.

I did some quick tests with schottky diodes in series with the PV cell.

Without the diode I got ~0.7uA reverse leakage current from the batteries through the PV cell when the cell was completely covered.

I tested the following diodes that was readily available in my supply:

PMEG3050EP 30V
SK14B 40V
10BQ040 40V
SD103AW-V-GS08 40V

The PMEG3050 did not reduce the reverse leakage current. The SD103AW reduced the leakage current to ~0.25uA and the others where somewhere inbetween.

Forward voltage drop was less than 100mV at the ~4mA charge current for all diodes (@battery voltage ~1.3V).

So I guess this means that the diode is not needed in my circuit.

Perhaps the inherent diode in the cell has a larger forward voltage drop than the batteries will supply so the current is not drained from the battery even when the PV cell is not generating voltage/current?

I guess it would be cruicial for the original design if the panel discharges the battery at the same time as the LED is on.

> A series reverse diode or equivalent is, alas, essential.
> It's easy to not realise that they work "somewhat strangely" compared
> to what may be logically expected.
>
> Photovoltaic cells are diodes, but in photo voltaic generation mode
> the Anode is positive relative to the Cathode so current flow when
> delivering energy is against the flow that the diode will usually
> permit. It makes sense when you look at the underlying theory but
> Murphy had fun here.
>
> The effect on the panels may be minimal depending on dissipation. They
> may well survive OK in low energy systems.
> BUT this will be fatal for battery stored energy.
> When PV panel voltage falls below battery voltage the battery *will*
> discharge into the panel.
>
> Here's a basic tutorial on what is involved.
>
> http://cnx.org/content/m11343/latest/
>
> When using low voltage low current panels such as in path lights, use
> of a 1A Schottky at as low a reverse voltage as is sensibly available
> is liable to pay (very) slight dividends.
> A 1N5817 / SS12 diode is 20V rated (afair) and the 1N5818 / SS14 and
> 1N5819 are higher (30V, 40V, ...). They usually cost the same.
> Often but not always the lower Vr diode will have usefully lower
> Vforward - maybe more than 0.1V lower.
> When you have lots of light and Vpv >> Vbattery it matters little but
> as Vpv approaches Vbattery an extra 0.1V can help. Albeit, only
> sometimes.
>
> In the unlikely event that MPPT is being used, in a 3V system a 0.1V
> drop may give an extra 3% energy.
>
> Usually other factors swamp such small gains.
>
> Russell McMahon

> Instead of using a Schottky diode, why not use a P-MOSFET? the intrinsic
> diode will have a much lower voltage drop I suspect the resistance of the
> MOSFET if carefully chosen would be lower.
>
> Colin
> --

Do you mean like in a reverse voltage polarity protection? Or just using the intrinsic diode (gate tied to source)?

The reverse voltage polarity protection circuit will not work since I have the battery on one side of the p-mos and the PV cell on the other. Or?

> Without the diode I got ~0.7uA reverse leakage current from the batteries
> through the PV cell when the cell was completely covered.

- I have seen many many many PV designs of a very wide range of sizes.
Every one uses a blocking diode.

- When connected normally the PV cells are NOT reverse biased by the
battery when the PV panel is "dark" - they are FORWARD biased by the
battery. It is conceivable that you have a battery voltage that is low
enough to not matter about the forward current when the panel is dark.
However, this effect is not relied on in any design I have seen.

As a guide, a 12 cell 6V, 100 mA monocrystalline silicon panel that I
just tested has the following characteristics.
Panel dark, power supply connected positive to positive, as in normal operation.

Vsupply / Current in microamps

1 7
2 27
3 80
4 300
5 1000 = 1 mA
6 3 ma
6.5 10 mA
7 26 mA

This panel has a rated loaded voltage of about 6V.
It charges 3 NimH cells of about 4V Voc when fully charged.
So the battery would discharge at about 0.3 mA into the dark panel.
This level of discharge MAY be acceptable depending on other criteria.

When fully discharged the battery would be about 3V and providing about 80 uA.

Even a very small amount of light on the panel would bring the voltage
up to at least part of maximum and further reduce the discharge
current.

In most cases (see my prior comments) there is no great gain energy
wise in eliminating the diode as the panels run essentially as close
to constant current sources .

Battery discharge current will be reduced by any illumination of the panel.

As a guide, under fluorescent lighting on a workbench the above 6V,
100 mA panel produces 3V o/c and about 100 uA s/c. I'd estimate
lighting level (not tested) to be about 100 lux.

Increasing the number of PV cells per battery-cell would reduce the
dark discharge current but not make economic sense.

>
> I tested the following diodes that was readily available in my supply:
>
> PMEG3050EP 30V
> SK14B 40V
> 10BQ040 40V
> SD103AW-V-GS08 40V
>
> The PMEG3050 did not reduce the reverse leakage current. The SD103AW reduced
> the leakage current to ~0.25uA and the others where somewhere inbetween.
>
> Forward voltage drop was less than 100mV at the ~4mA charge current for all
> diodes (@battery voltage ~1.3V).
>
> So I guess this means that the diode is not needed in my circuit.
>
> Perhaps the inherent diode in the cell has a larger forward voltage drop than
> the batteries will supply so the current is not drained from the battery even
> when the PV cell is not generating voltage/current?
>
> I guess it would be cruicial for the original design if the panel discharges

At 06:48 PM 5/7/2012, Robert Rolf wrote:
>PV cells are DIODES. There is no need for an additional one to 'isolate' it
>from the battery.
>
>On Mon, May 7, 2012 at 5:23 PM, doug metzler <doug.metzlerEraseME.....gmail.com> wrote:
>
> > Based on the google research I did before I built the circuit it was
> > consensus that it's probably not good for the panels to have power
> > pushed back into them at night so I went ahead and added the schottky
> > diode.

The testing that I did a number of years ago showed that the PV cells I was working with had significant reverse leakage when dark. We found it necessary to add a series Schottky diode to reduce that leakage (the Schottky diode also had reverse leakage but it was at least an order of magnitude less than the PV cell).

Modern PV cells may no longer exhibit large reverse currents when dark - I simply don't know. But it certainly would be easy to test for that and add the diode if needed.

FTW - I found that the extra voltage drop introduced by the series diode had negligible effect on the light level required for charging the batteries. This was on both 6V and 12V PV systems (relatively small - 5 to 10W).

At 04:41 AM 5/8/2012, cdb wrote:
>Instead of using a Schottky diode, why not use a P-MOSFET? the intrinsic
>diode will have a much lower voltage drop I suspect the resistance of the
>MOSFET if carefully chosen would be lower.

This will work but you need active electronics to drive the gate of the MOSFET.

One of my solar charger designs does exactly this - it uses a current sensor driving a comparitor that drives the MOSFET's gate.

The MOSFET is connected in reverse so that the intrinsic back-diode conducts when the solar voltage is high enough. Above a certain current flow, the current sensor trips the comparitor which then turns the MOSFET hard ON. When the current drops below the threshold, the MOSFET turns back off.

This was in a high-Arctic application where I wanted to grab every photon that was available in the winter months. The rest of the system was designed to function without sunlight for a 3-month period. After more than 6 or 7 years in operation, all units are still functioning correctly.

"=?ISO-8859-1?Q?Ruben_J=F6nsson?=" writes:
> > PV cells are DIODES. There is no need for an additional one to
> 'isolate' it
> > from the battery.
> >
>
> Ok, so no diode is needed to not hurt the cells.
>
> What about reverse leakage current?
>
> You don't want to discharge the batteries through the cells when it is
> dark.

If you leave the diode out of some arrays, it only gets
truly dark when the battery discharges.:-)

Somewhere, I read an account of a PV pannel at some
experimental site that exhibited LED properties when the
batteries tried to discharge back through the Solar cells.
It was reported that the cells gave off an eerie blueish-purple
glow.

> BCCs: Worth noting.
>
> > Without the diode I got ~0.7uA reverse leakage current from the batteries
> > through the PV cell when the cell was completely covered.
>
> - I have seen many many many PV designs of a very wide range of sizes.
> Every one uses a blocking diode.
>
> - When connected normally the PV cells are NOT reverse biased by the
> battery when the PV panel is "dark" - they are FORWARD biased by the
> battery. It is conceivable that you have a battery voltage that is low

Yes, I understand that now. Reverse leakage current is the wrong term here.

> enough to not matter about the forward current when the panel is dark.
> However, this effect is not relied on in any design I have seen.

The original design does not have a diode between the PV cell and the NiMh cell. There is one PV cell for one NiMh cell so the nominal dark voltage over the PV cell from the battery is 1.2V.

Perhaps there is a diode built in into the PV cell or its fixture. I have not removed the PV cell from the original casing.

> When fully discharged the battery would be about 3V and providing about 80 uA.
>
> Even a very small amount of light on the panel would bring the voltage
> up to at least part of maximum and further reduce the discharge
> current.
>
> In most cases (see my prior comments) there is no great gain energy
> wise in eliminating the diode as the panels run essentially as close
> to constant current sources .
>
> Battery discharge current will be reduced by any illumination of the panel.
>
> As a guide, under fluorescent lighting on a workbench the above 6V,
> 100 mA panel produces 3V o/c and about 100 uA s/c. I'd estimate
> lighting level (not tested) to be about 100 lux.
>
> Increasing the number of PV cells per battery-cell would reduce the
> dark discharge current but not make economic sense.
>
>
> Russell
>

Thanks for these comments. I still learn something new on the piclist, even after being a member for around 15 years.